1. Field of the Invention
This invention relates to a power converter apparatus or an inverter apparatus for changing DC power into AC power. In addition, this invention relates to a method of controlling the drive of a polyphase AC-powered load via a power converter apparatus or an inverter apparatus.
2. Description of the Related Art
In general, an inverter apparatus or a power converter apparatus is provided between a DC power supply and an AC-powered load. The inverter apparatus receives DC power from the DC power supply. The inverter apparatus changes the DC power into AC power. The inverter apparatus feeds the AC power to the AC-powered load, and thereby drives the AC-powered load.
A typical power converter apparatus for a polyphase AC motor includes IGBT's (insulated gate bipolar transistors) as switching elements. Switching action of each of the IGBT's causes a power loss called a switching loss. As the frequency of switching rises, the switching loss increases. It is desirable to drive the polyphase AC motor while minimizing the switching losses caused by the IGBT's.
Japanese published unexamined patent application 59-139871 (application number 58-11678) discloses a pulse-width modulation system for the drive of a three-phase AC motor. In the system of Japanese application 59-139871, two-phase modulation is implemented. Specifically, the system includes three unit inverters for the three phases respectively. During every one sixth of a period (a cycle), one of the three unit inverters is controlled to fix a corresponding phase voltage while the two other unit inverters are controlled to conform the corresponding inter-phase voltages to sinusoids. Thus, a motor drive signal for one of the three phases is non-modulated while motor drive signals for the two other phases are modulated in accordance with the sinusoids. Selection of one from among the three unit inverters as corresponding to a non-modulated phase is designed to reduce the number of times of commutations per cycle, and hence to decrease commutation losses.